Method of producing ball valve

ABSTRACT

A ball valve is produced, which has a valve housing  10  and a ball  20 . A pair of mirror-symmetrical housing halves  12, 14  are jointed together along a single weld line. The housing halves are jointed together by welding in a manner to enclose the ball previously provided with a through bore and a pair of recessed portions. The through bore  22  of the ball is arranged to be in conformity with the weld line. Holes  42, 50  for respectively mounting a pair of stem assemblies are formed by machining respectively in those portions of the valve housing opposed respectively to open opposite ends of the through bore. The centers of the holes are in alignment with the operational centerline of the ball valve. The ball is turned by an angle of 90 degrees, thereby bringing the through bore of the ball into alignment with openings  12 A,  14 A of the valve housing, and the recessed portions  26, 28  of the ball are brought into alignment with the holes, respectively. Next, the pair of stem assemblies  60, 68  are attached to the both holes, respectively, and stems are fitted in the both recessed portions, respectively, and the pair of stems are welded circumferentially to peripheral edge portions of the holes, respectively.

TECHNICAL FIELD

[0001] The present invention relates to a method of producing a ballvalve to be connected to a fluid piping system, and more particularly toa method of assembling a housing, adapted to contain a ball therein, bywelding.

BACKGROUND ART

[0002] (1) U.S. Pat. No. 4,265,427 (corresponding to JP-A-55-14394)discloses one example ball valve which comprises, as main componentmembers, a ball, and a housing containing the ball.

[0003] In the ball valve, a spherical shut-off member (i.e. a ball),contained in a hood (or a housing), has a pair of projections (orstems), and the projections are rotatably supported by a pair of supportrings attached to an inner surface of the hood, respectively. The hoodis assembled by welding a pair of hood halves together, and butt weldingend faces of the pair of hood halves lie in a plane including an axis ofrotation of the spherical shut-off member. Namely, the pair of hoodhalves are mirror-symmetrical with respect to the plane including theaxis of rotation of the spherical shut-off member.

[0004] The hood has an operation hole (or window) provided for rotatingthe spherical shut-off member around the axis, and hemi-circularrecesses, corresponding to the operation hole, are formed respectivelyin the pair of hood halves before the two hood halves are jointedtogether by welding.

[0005] The pair of support rings serve to receive a thrust force from afluid, are provided respectively at opposed positions on the innersurface of the hood, and are disposed near to the axis of rotation ofthe spherical shut-off member in surrounding relation thereto, whereinone of the support rings surrounds the operation hole. Each of the twosupport rings comprises a combination of support ring halvescorrespending respectively to the pair of hood halves, and the supportring halves are secured respectively on the inner surfaces of the pairof hood halves by welding prior to welding of the hood halves.Therefore, when the pair of hood halve are jointed together by welding,each pair of support ring halves are abutted against each other at theirbutting end faces to form the circular ring.

[0006] In order to prevent the inside of the hood from being affected bythe welding operation (by which the pair of hood halves are jointedtogether), a partially cut off grooved ring is attached by welding onthe inner surface adjacent to the welding face (i.e. the butt weldingend face) of one of the hood halves. The grooved ring extends toward themating hood half in the direction of the width thereof so as to extendcircumferentially along the inner side of the butted end faces of thepair of hood halves for welding.

[0007] In the ball valve having such a structure, since the pair ofsupport rings, which receive a thrust force from fluid, and theoperation hole of the hood, are located close to the butt welded facesof the pair of hood halves, there is a possibility that thermal strainis induced in the hood due to welding heat resulting in a displacementof each support ring half from its proper position. In this case, theprecision of the axis of rotation of the spherical shut-off member isaffected, so that incomplete contact between valve seats, which areprovided on the inner surface of the hood, and the spherical shut-offmember occurs.

[0008] (2) U.S. Pat. No. 4,235,003 discloses a method of producing aball valve. In the disclosed ball valve, a ball is contained in a hollowmember (i.e. a housing), and the ball is rotatably supported by a pairof rings (i.e. valves seats). In the ball valve having such structure,unlike the ball valve disclosed in U.S. Pat. No. 4,265,427, the ringsreceive a thrust force from fluid.

[0009] The hollow member of the ball valve, disclosed in U.S. Pat. No.4,235,003, is formed by jointing a pair of symmetrical pipe memberstogether by welding. Each pipe member has a hemi-spherical enlargedportion (of a bowl-shape) formed at one end thereof, and open end facesof the two enlarged portions are butted and welded together at the endfaces, thereby forming the hollow member. When the ball valve is openedor closed, the ball within the hollow member is operated by an operationstem passing through a hole formed in the hollow member, the hole beingformed by a combination of hemi-circular recesses which are formedrespectively in the open ends of the two enlarged portions prior to thewelding for obtaining the hollow member. Here, the remaining portion ofeach pipe other than the enlarged portion will be referred to as astraight pipe portion, and a sleeve of a wedge-shaped cross-section forholding a ring (i.e. a valve seat) is fitted into each of the straightpipe portion, and is fixedly secured thereto by welding.

[0010] In the ball valve, although the straight pipe portions have aninner diameter corresponding to that in a fluid piping, there is adisadvantage that the sleeves and the rings (i.e. valve seats) narrow afluid passage. Further, since the hemi-circular recesses are formedrespectively in the enlarged portions of the two pipe members prior towelding for obtaining the hollow member, there is a possibility that theoperating stem-passing hole, formed by the combination of thehemi-circular recesses, is affected by thermal strain which is inducedin the hollow member by welding, so that the center of the hole isdisplaced out of a proper position.

[0011] (3) A third example of known ball valves is shown in FIG. 5. Theball valve comprises, as main components, a housing 01, having openings02A and 03A formed respectively at opposite sides thereof, a ball 05,which is contained in the housing, and has a central through bore 05A,and a pair of stems 07 and 08 supporting the ball. The ball 05 isrotated by the operation stem 08 about the axis of the both stems. Thereare provided valve seat rings 06 and 06 between the housing 01 and theball 05 which is kept always in contact with the valve seat rings 06 and06.

[0012] The housing 01 consists of three members, that is, a first member02, a second member 03 and a third member 04, the first and secondmembers being mirror-symmetrical. The three members each having agenerally tubular shape are butted and welded (W) at their open endfaces.

[0013] The stems 07 and 08 extend through the third member 04. The stem07 extends through the third member 04 and is fixed thereto. It is alsorotatably fitted in a bore for stem of the ball 05. The other stem 08 isof an operation stem for rotating the ball valve, which rotatablyextends through the third member 04, is supported by a separate supportmember 09, and is fitted in a bore for stem of the ball 05 so as not torotate relatively to the ball.

[0014] This ball valve is of such a structure that the pair of stems 07and 08 receive a thrust force from fluid. The housing 01 is assembled byjointing the three sectional members together by welding, which isdisadvantageous from an economical point of view since the number of thecomponents is large. Besides, since the two weld lines W, W are close tothe valve seat rings 06 and 06, respectively, there will occur anunconformity with respect to the contact relationship between each ofthe valve seat rings 06, 06 and the ball 05 under the influence ofthermal strain induced in the housing. Further, since the weld lines W,W are close to the stems 07 and 08, the shape of the respective holes ofthe third member 04 for the stems is affected by the thermal strain,whereby the precision of the stems 07 and 08, that is, the axis ofrotation of the ball 05, is deteriorated, also resulting in anunconformity with respect to the contact relationship between each ofthe valve seat rings 06, 06 and the ball 05.

[0015] The present invention has been proposed under the above technicalbackground.

[0016] Problems to be solved by the invention are to reduce the numberof components, in producing a so-called trunnion type ball valve, inorder to save the production cost for the trunnion type ball valve, suchas the first conventional example of U.S. Pat. No. 4,265,427 and thethird conventional example shown in the accompanying drawing of FIG. 5,and to eliminate adverse effects of thermal strain induced in a housingdue to welding whereby obtaining the ball valve with a high precision.

DISCLOSURE OF THE INVENTION

[0017] According to the present invention, there is provided thefollowing ball valve producing method:

[0018] a ball valve to be produced is connected to a fluid pipingsystem, and comprises a valve housing (for example, a wall thickness=10to 50 mm), and a ball rotatably contained in the valve housing. Thevalve housing and the ball are made, for example, of carbon steel oraustenitic stainless steel (JIS SUS304).

[0019] The valve housing has a pair of holes, which are located inopposed positions so as to be connected respectively to pipesconstituting the piping system, and a pair of stem assemblies which arelocated in opposed positions on a straight line perpendicularlyintersecting another straight line, passing through centers of the pairof openings and a center of the ball, at the center of the ball.

[0020] The ball has a through bore, which can be brought into alignmentwith the pair of openings to allow the passage of a fluid therethrough,and a pair of recessed portions receiving the pair of stem assemblies,respectively. The ball is rotatable within the valve housing about acenterline passing through the pair of stem assemblies.

[0021] The ball valve having the above structure is produced by thefollowing process:

[0022] (1) A pair of housing halves for forming the valve housing, andthe ball having the through bore whose opposite open ends are formed tobe flat, are prepared. The pair of housing halves are so formed to bemirror-symmetrical bodies of which forms are defined by sectioning thevalve housing at the middle between the pair of openings. The ball ispreviously provided with a pair of recessed portions.

[0023] (2) Annular valve seats, each forming a seal between an outersurface of the ball and an inner surface of the valve housing, aremounted in the pair of housing halves, respectively.

[0024] (3) The pair of housing halves are butted at their open ends(opposite to the openings, respectively) so as to enclose the ball. Atthis time, the ball is kept in such a position that the ball is turnedby an angle of 90 degrees from a normal operational position of the ballvalve, while causing the center-line of the through bore to be inalignment with the operational axis of the ball and to perpendicularlyintersect the straight line passing through the centers of the pair ofopenings and the center of the ball.

[0025] (4) The butted open ends of the pair of housing halves arejointed together by welding, thereby forming the valve housing.

[0026] (5) Without changing the position of the ball, holes for mountingthe pair of stem assemblies respectively, are formed by machining atthose positions of the valve housing faced respectively to the both flatopen ends of the through bore of the ball. The both holes are so formedthat the centers thereof are located on the operational centerline ofthe ball valve.

[0027] (6) The ball is turned by an angle of 90 degrees to be in thenormal operational state of the ball valve such that the through bore isaligned with the both holes of the valve housing and that the recessedportions of the ball are aligned with the both holes of the housing,respectively. Next, the pair of stem assemblies are attached to the bothholes, respectively, and stems are fitted into the both recessedportions, respectively, and subsequently the pair of stem assemblies arewelded circumferentially to edge portions of the holes, respectively.

[0028] An embodiment of the invention method characterized by the abovesteps of the process will be described in the following:

[0029] (1) The welding at step (4) is effected by a TIG welding method(i.e. Tungsten inert-gas arc welding). In the TIG welding method, sinceno flux is used, spattering of slag will not occur during the weldingoperation, so that the contamination of the interior of the valvehousing by foreign matters can be effectively prevented. Here, it shouldbe noted that the entry of foreign matters, which would adversely affectthe condition of contact of the ball with the valve seats, should bepositively avoided.

[0030] (2) One of the pair of stem assemblies includes a non-operationstem, fitted in one of the two recessed portions, and an auxiliarymember, and a base portion of the auxiliary member is weldedcircumferentially by welding to the peripheral edge of one of the holes.The other of the pair of stem assemblies includes a sleeve, fixedlysecured to the valve housing, and a valve stem (i.e. valve-operatingstem) rotatably fitted in the sleeve, with a seal ring interposedtherebetween, and in a condition in which the valve stem is fitted inthe sleeve, a distal end portion of the valve stem is fitted in theother recessed portion in a manner to prevent a relative rotationtherebetween, and an end portion of the sleeve is jointed by welding tothe peripheral edge of the hole over an entire periphery of the sleeveend portion.

[0031] (3) A flange plate is detachably fixed to a free end of thesleeve to prevent the valve stem from moving axially out of engagementwith the sleeve, and a free end portion of the valve stem extendsoutwardly from the flange plate through an opening in the flange plate.This outwardly-extending portion is used as a valve-operation stem forrotating the ball about the centerline of the ball to open and close theball valve.

[0032] (4) The pair of valve seats are mounted respectively on the innersurfaces of the housing halves in such a manner that the valve seats arelocated close to the pair of openings in parallel with end surfaces ofthe openings, respectively.

[0033] (5) Each of the pair of valve seats has a resilient member, andthe valve seat is brought to close contact with the outer surface of theball by a spring force of the resilient member.

[0034] (6) A body of the valve seat is a metal ring, and a resin ring ismounted on a peripheral edge of the metal ring, and the resin ring isbrought to close contact with the outer surface of the ball.

[0035] (7) A body of the valve seat is a metal ring, and a rubber ringis mounted on a peripheral edge of the metal ring, and the rubber ringis brought to close contact with the outer surface of the ball.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036]FIG. 1 is a cross-sectional view of a ball valve through an axisthereof, which is an intermediate product produced by the inventionmethod;

[0037]FIG. 2 is a cross-sectional view of the ball valve through theaxis thereof, which is produced by the invention method;

[0038]FIG. 3 is a cross-sectional view of a part of a valve housing,explanatory of a preparatory step for forming a hole of the valvehousing after welding according to the invention method;

[0039]FIG. 4 is a cross-sectional view of a part of the valve housing,explanatory of a preparatory step subsequent to the step of forming aprovisional hole in the valve housing after welding according to theinvention method; and

[0040]FIG. 5 is a cross-sectional view of a conventional ball valve.

BEST MODE FOR CARRYING OUT THE INVENTION

[0041] An embodiment of the present invention will now be described withreference to the drawings.

[0042]FIG. 1 shows a cross-section of a ball valve (as an intermediateproduct) through an axis thereof. FIG. 2 is a cross-sectional view ofthe assembled ball valve.

[0043] First, the structure of the ball valve will be described. A ball20 is rotatably contained within a valve housing 10 having a generallyhollow cylindrical shape. The valve housing 10 has opposite open ends(openings 12A and 14A), and has a first hole 42 and a second hole 50formed respectively through opposed portions of a central portion of thehousing body (FIG. 1). The ball valve is connected to a piping system atthe circular openings 12A and 14A of the valve housing 10. As shown inFIG. 2, a first stem portion 60 and a second stem portion 68 are fixedlysecured respectively to the first hole 42 and the second hole 50 bywelding.

[0044] Valve Housing

[0045] The body of the valve housing 10 has such a hollow cylindricalshape that the body is increasing in diameter from the circular openings12A and 14A, formed respectively at the opposite ends thereof, toward acentral portion thereof where the first hole 42 and the second hole 50,closed respectively by the first stem 60 and the second stem 50, exist.

[0046] Annular Valve Seats

[0047] A pair of annular valve seats 30 and 30 are provided on an innersurface of the valve housing 10 to form a seal between the valve housing10 and the ball 20. The ball 20 can be rotated while kept in contactwith the annular valve seats 30 and 30. The annular valve seat 30comprises a metal ring 32, serving as a base portion, a plurality ofcompression coil springs 36 (preferably, ten compression coil springs),a sealing material ring 34 (For example, this is made of a resinmaterial or a rubber material), mounted on a distal end of the metalring 32 abutted against the ball 20, and an O-ring interposed betweenthe metal ring 32 and the valve housing 10. The metal ring 32 and thesealing material ring 34 are pressed against a spherical outer surfaceof the ball 20 by the bias of the compression coil springs 36.

[0048] Ball

[0049] By removing portions of the ball 20, this ball is formed intosuch a configuration that parallel open ends 24 and 24 are formedrespectively at opposite sides of the spherical body thereof, and theball 20 has a central through bore 22, a first recessed portion 26 and asecond recessed portion 28 all of which are cross-sectionally circular.A key groove is formed in the second recessed portion 28. The firstrecessed portion 26 and the second recessed portion 28 are formedrespectively in opposite portions of the surface of the ball 20, and astraight line L1, passing through the centers of these recessedportions, perpendicularly intersects a centerline (that is, an axis) L2of the central through bore 22 (FIG. 2). In the condition shown in FIG.2, the centerline of the central through bore 22 is in alignment withthe centerline of the valve housing 10, that is, a straight line (i.e.the axis) passing through the centers of the circular openings 12A and14A.

[0050] Stem Portions

[0051] The first stem portion 60 includes a base portion 62 (auxiliarymember), and a first stem 64 (non-operating stem) in the form of a roundrod. The base portion 62 is fitted in the first hole 42 in the valvehousing 10, and is welded and fixed to a peripheral edge of the firsthole. The first stem 64 is fitted in the first recessed portion 26 ofthe ball 20.

[0052] The second stem portion 68 comprises a cylindrical sleeve 70, avalve-operating valve stem 72 (in the form of a round rod), fitted inthis sleeve so as to rotate relative thereto, and a flange plate 74secured to an outer end of the sleeve 70 by screws. One end portion ofthe sleeve 70 is fitted in the second hole 50 in the valve housing 10,and is welded and fixed to a peripheral edge of the second hole. Thevalve stem 72 projects from the one end of the sleeve 70, and is fittedin the second recessed portion 28 of the ball 20. A key 28A is engagedin key grooves, formed respectively in the valve stem 72 and the secondrecessed portion 28, thereby preventing a relative rotation between thetwo members 72 and 28. An O-ring 76 forms a seal between the valve stem72 and the sleeve 70. That portion of the valve stem 72, extendingoutwardly from the sleeve 70, is smaller in diameter than its bodyportion fitted in the sleeve 70, and this smaller-diameter portionextends through an opening, formed through the flange plate 74, and astep portion, formed between the larger-diameter portion andsmaller-diameter portion of the valve stem 72, is engaged with theflange plate 74, thereby preventing the movement of the valve stem 72 inthe axial direction. The axes of the first stem 64 and the valve stem 72lie on the straight line L1 (FIG. 2), and by rotating thesmaller-diameter portion of the valve stem 72, the ball 20 can berotated about the straight line Li so as to open and close the ballvalve.

[0053] Next, a method of producing the ball valve will be described.

[0054] (1) Preparation of Valve Housing Constituent Members (FIG. 1):

[0055] A pair of housing halves 12 and 14 for forming the valve housing10 are prepared. The housing halves 12 and 14 are mirror-symmetricalwith respect to a plane (reference plane) including the common axis ofthe first and second holes 42 and 50, formed in the valve housing 10,and the center C of the ball. The housing half 12 are open at itsopposite ends, and has the opening 12A and a larger-diameter end 12B.Similarly, the housing half 14 has the opening 14A and a larger-diameterend 14B. The annular valve seats 30 are mounted in the housing halves 12and 14, respectively. This mounting relation will be described withrespect to the housing half 12, and a step portion is formed on thatportion of the inner surface of the housing half 12, disposed near tothe opening 12A of the housing half 12, in parallel relation to thisopening 12A defining a flat opening lying in a plane, and the annularvalve seat 30 is inserted into the housing half 12 through thelarger-diameter end 12B, and is mounted on this step portion in fittedrelation thereto. Similarly, the annular valve seat 30 is mounted on thehousing half 14. As described above, each of the annular valve seats 30and 30 serves to form a seal between the outer surface of the ball andthe inner surface of the valve housing.

[0056] (2) Preparation of Ball (FIG. 1):

[0057] There is prepared the ball 20 which includes the central throughbore 22 for allowing the flow of a fluid, and the opposite open ends 24and 24 which are flat and parallel to each other. The first recessedportion 26 and the second recessed portion 28 are preformed in the ball20 by machining. These recessed portions lie on the common axis.

[0058] (3) Combination of Housing Halves with Ball:

[0059] The housing halves 12 and 14 are butted at their open ends 12Band 14B (opposite with respect to the openings 12A and 14A,respectively) in a manner to enclose the ball 20. At this time, the ball20 is kept in such a position that the direction of the ball 20 isturned by an angle of 90 degrees with respect to a normal ball valveoperational condition, with the centerline of the through bore 22coinciding with the centerline of the ball 20 (about which the ball isopened and closed), and perpendicularly intersecting the straight linepassing through the centers of the openings 12A and 14A and the centerof the ball 20.

[0060] (4) Welding:

[0061] The butted open ends (larger-diameter ends) 12B and 14B of thehousing halves 12 and 14 are jointed together by welding (W), therebyforming the valve housing 10. This welding is effected by a TIG weldingmethod. With this TIG welding, slag will not be scattered over theinterior of the valve housing 10 during the welding, and this can serveto ensure the performance of the ball valve. In a preferred practicalmethod, first to third layers of weld beads are formed by TIG welding,and subsequent bead layers are formed by an MIG welding method (Inertgas arc welding using a consumable electrode) or a submerged arc method.

[0062] (5) Formation of Holes:

[0063] While keeping the ball 20 in the position described in the aboveItem 3, the pair of holes (the first hole 42 and the second hole 50) forrespectively mounting the stem assemblies 60 and 68 are formed bymachining respectively in those portions of the valve housing opposedrespectively to the flat open ends 24 and 24 of the through bore 22. Atthis time, the centers of the two holes to be formed lie on theoperational centerline of the ball valve about which the ball is openedand closed.

[0064] Specifically, the holes are formed according to the followingprocedure (see FIGS. 3 and 4).

[0065]FIG. 1 shows a condition in which the first hole 42 and the secondhole 50 are already formed, but description will start from thecondition in which the valve housing is as welding, and the holes arenot yet formed. The two holes are formed by the same method, andtherefore only the method of forming the first hole 42 will bedescribed, and description of the method of forming the second hole 50will be omitted.

[0066] a. That portion, corresponding to the first hole 42, is cut to beformed into a flat surface 40, as shown in FIG. 3.

[0067] b. Screw holes 44 and 44 are formed respectively in positionswhich are point-symmetrical with respect to the imaginary center of thefirst hole 42 disposed on the weld line W, and screws, each having ahook, are threaded into the screw holes 44 and 44, respectively.

[0068] c. An annular groove 46 is formed in surrounding relation to thescrew holes 44 and 44, and is spaced outwardly from these screw holes.The depth of the annular groove 46 is slightly smaller than the wallthickness of the relevant portion, and the remaining wall thickness is aminimum dimension which can support the weight of a disk portion definedby the annular groove 46.

[0069] d. The disk portion, formed inside the annular groove 46, ispulled hard outwardly, using the pair of screws (each having the hook)threaded respectively in the screw holes 44 and 44, thereby rupturingthe thin wall portion remaining at the bottom of the annular groove 46.As a result of this operation, a provisional hole 48 is formed (FIG. 4).

[0070] e. Utilizing the thus formed provisional hole 48, an annularfoamed resin 52 is interposed between the open end 24 of the throughbore of the ball 20 and the inner surface of the valve housing 10.

[0071] f. A peripheral edge portion of the provisional hole 48 isremoved by cutting, thereby forming the first hole 42 (see broken linesin FIG. 4) of the predetermined size. At this time, because of existenceof the annular foamed resin 52, cuttings, produced when cutting theperipheral edge portion of the provisional hole 48, will not enterspaces in which the spherical surface portions of the ball 20 aredisposed, respectively. Incidentally, if the above steps a to e areeffected for the second hole 50 before the peripheral edge portion ofthe provisional hole 48 is removed by cutting, the influence of thecuttings on the spherical surface portions of the ball 20 can beprevented more positively. After the peripheral edge portion of theprovisional hole 48 is removed by cutting, the inner surface of thethrough bore 22 is cleaned, and the annular foamed resin 52 is removed.

[0072] (6) Welding of Stem Portions

[0073] After the operations of the above five Items are finished, theball 20 is turned by an angle of 90 degrees, and is brought into theposition shown in FIG. 2. In this ball position, the first stem portion60 and the second stem portion 68 are disposed in alignment with thefirst hole 42 and the second hole 50, respectively.

[0074] a. First Stem portion 60: The base portion 62 and the first stem64 are inserted into the first hole 42 and the first recessed portion26, respectively. The base portion 62 is larger in diameter than thestem 64. The peripheral surface of the base portion 62 is jointed to theperipheral edge portion of the first hole 42 by TIG welding (W).

[0075] b. Second Stem portion 68: The valve stem 72 and the sleeve 70are previously assembled together. The distal end portion of the valvestem 72, projecting from the sleeve 70, is inserted into the second hole50. At this time, the key 28A is engaged in the key groove, formed inthe second hole 50, and the key groove formed in the valve stem 72, toprevent the valve stem 72 from rotating in the second hole 50.Subsequently, while the valve stem 72 is mounted in the second hole 50,the outer surface of the sleeve 70 is jointed to the peripheral edgeportion of the second hole 50 by TIG welding (W). By effecting thewelding in the mounted condition of the valve stem 72, the alignment ofthe sleeve 70 at the time of the welding can be effected accurately.

[0076] When the welding of the stems is effected, the assemblage of theball valve 10 is completed. Thereafter, pipes of the fluid piping systemare connected respectively to the openings 12A and 14A of the ball valve10.

[0077] Advantages of the present invention are as follows:

[0078] (1) In the present invention, the valve housing is formed bywelding the pair of housing halves together, and the number of thecomponent parts are smaller as compared with the conventionaltrunnion-type ball valve (shown in FIG. 5) in which the valve housing isformed by the three housing members. And besides, the welding portion,required for assembling the valve housing, is one, and therefore theproduction cost can be reduced.

[0079] (2) The weld line, formed at the time of assembling the valvehousing, can be disposed at the central position of the valve housingsufficiently spaced from the valve seats, and therefore the influence ofthe welding heat on the valve seats is small, and this can contribute tothe enhanced precision of the ball valve.

[0080] (3) The pair of holes for respectively mounting the stemassemblies are formed after the housing halves are jointed together bywelding, and therefore in contrast with the case where the holes arepreviously formed, and therefore the holes are not subjected to theinfluence of thermal strain due to the welding, so that the axes of thestems can be set highly precisely.

[0081] (4) When forming the pair of holes after the valve housing isassembled by jointing the housing halves together by welding, theopposite open ends of the through bore of the ball (which is rotatablewithin the valve housing) are disposed in alignment with the pair ofholes, respectively, and by doing so, the influence of cuttings, formedat the time of formation of the holes, on the spherical surface of theball and the valve seats, can be prevented. And, after the holes areformed, the ball can be returned into the normally-used condition byrotating this ball by the angle of 90 degrees, and therefore this isconvenient.

[0082] (5) In the ball valve formed in accordance with the presentinvention, the valve seats are previously mounted within the housinghalves, respectively, and the ball is received, and the valve housing isassembled (by welding), and therefore in contrast with the ball valve,disclosed in U.S. Pat. No. 4,235,003 (in which the valve housing isassembled after putting a ball in the housing, thereafter the valveseats are inserted and fixed to the valve housing by welding), thethrough bore of the ball is sufficiently large, and can be formed intothe same diameter as the diameter of the fluid inlet and outlet openingsin the valve housing, and the ball valve will not offer a resistance tothe flow of the fluid (that is, will not disturb a stream line of thefluid flow).

[0083] Capability of Exploitation in Industry

[0084] The method of the present invention can be applied to bothsmall-size and large-size ball valves, and is suited particularly forthe production of a large-size ball valve incorporated in a pipeline forpetroleum or natural gas.

1. A method of producing a ball valve which is adapted to be connectedto a fluid piping system, and comprises a valve housing, and a ballrotatably contained in the valve housing, wherein the valve housing hasa pair of openings, which are located in opposed positions so as to beconnected respectively to pipes constituting the piping system, and apair of stem assemblies which are located in opposed positions on astraight line perpendicularly intersecting another straight line,passing through centers of the pair of openings and a center of theball, at the center of the ballthethethethe; and wherein the ball has athrough bore, which can be brought into alignment with the pair ofopenings to allow the passage of a fluid therethrough, and a pair ofrecessed portions receiving the pair of stem assemblies, respectively,and the ball is rotatable within the valve housing about a centerlinepassing through the pair of stem assemblies, characterized in that: apair of housing halves for forming the valve housing, and the ballhaving the through bore whose opposite open ends are formed to be flat,are prepared, the pair of housing halves being so formed to bemirror-symmetrical bodies of which forms are defined by sectioning thevalve housing at the middle between the pair of openings, and the ballbeing previously provided with a pair of recessed portions; annularvalve seats, each forming a seal between an outer surface of the balland an inner surface of the valve housing, are mounted on the pair ofhousing halves, respectively; the pair of housing halves are butted attheir open ends, which are opposite to the openings, respectively, so asto enclose the ball, on the other hand, the ball is kept in such aposition that the ball is turned by an angle of 90 degrees from a normaloperational position of the ball valve, while causing the centerline ofthe through bore to be in alignment with the operational axis of theball and to perpendicularly intersect the straight line passing throughthe centers of the pair of openings and the center of the ball; thebutted open ends of the pair of housing halves are jointed together bywelding, thereby forming the valve housing; subsequently, withoutchanging the position of the ball, holes for mounting the pair of stemassemblies respectively, are formed by machining at those positions ofthe valve housing faced respectively to the both flat open ends of thethrough bore of the ball so that the centers thereof are located on theoperational centerline of the ball valve; and the ball is turned by anangle of 90 degrees to be in the normal operational state of the ballvalve such that the through bore is aligned with the both openings ofthe valve housing and that the recessed portions of the ball are alignedwith the both holes of the housing, respectively, next, the pair of stemassemblies are attached to the both holes, respectively, and stems arefitted into the both recessed portions, respectively, and subsequentlythe pair of stem assemblies are welded circumferentially to edgeportions of the holes, respectively.
 2. A ball valve-producing methodaccording to claim 1, wherein the step of jointing the open ends of thepair of housing halves together by welding is effected by a TIG weldingmethod.
 3. A ball valve-producing method according to claim 1, whereinone of the pair of stem assemblies has a non-operation stem, which isfitted in one of the recessed portions, and an auxiliary member, and abase portion of the auxiliary member is welded circumferentially bywelding to the peripheral edge of one of the holes; and the other of thepair of stem assemblies includes a sleeve, which is fixedly secured tothe valve housing, and a valve stem rotatably fitted in the sleeve, witha seal ring interposed therebetween, and a distal end portion of thevalve stem is fitted in the other recessed portion so as not torelatively rotate while the valve stem is fitted in the sleeve, and anend portion of the sleeve is welded circumferentially to the peripheraledge of the hole.
 4. A ball valve-producing method according to claim 1,wherein a flange plate is detachably fixed to a free end of the sleeveto prevent the valve stem from moving axially out of engagement with thesleeve, a free end portion of the valve stem extends outwardly from theflange plate through an opening of the flange plate, and theoutwardly-extending portion is used as a valve-operation stem forrotating the ball about the operational centerline of the ball to openand close the ball valve.
 5. A ball valve-producing method according toclaim 1, wherein the pair of valve seats are mounted respectively on theinner surfaces of the housing halves in such a manner that the valveseats are located close to the pair of openings in parallel with endsurfaces of the openings, respectively.
 6. A ball valve-producing methodaccording to claim 1, wherein each of the pair of valve seats has aresilient member, and the valve seat is brought to close contact withthe outer surface of the ball by a spring force of the resilient member.7. A ball valve-producing method according to claim 6, wherein a body ofthe valve seat is a metal ring, and a resin ring is mounted on aperipheral edge of the metal ring, and the resin ring is brought toclose contact with the outer surface of the ball.
 8. A ballvalve-producing method according to claim 6, wherein a body of the valveseat is a metal ring, and a rubber ring is mounted on a peripheral edgeof the metal ring, and the rubber ring is brought to close contact withthe outer surface of the ball.